LT_AT_EXEC_CHECK but uses `$LIBTOOL --mode=execute' and allows
to pass `-dlopen MODULE' arguments.
* tests/lt_dlexit.at: Use it. Also, do not test `-dlpreopen',
that currently needs library names to begin with `lib'.
before checking that it is still in the list.
* tests/lt_dlexit.at: New test.
* Makefile.am (TESTSUITE_AT): Adjust.
(check-local): Also depend on libltdl/libltdlc.la.
(check-recursive): Removed, unnecessary use of Automake
internals.